But unlike with past incidents, the Kremlin has remained largely silent on the attack, neglecting to name those killed or explain what they were doing at that location to begin with.

Various reports put the number killed in the hundreds, but Moscow has given far smaller estimates. The Kremlin's silence has led to friends and family of those killed to question why the government has seemingly ignored their service.

On the night of February 7, a 500-strong force largely made up of the Russian contractors and a Christian militia loyal to the Syrian regime crossed the River Euphrates near Deir Ezzor, a Syrian city held by ISIS until the end of last year.

The Russians were working for a paramilitary company called Wagner, which has hundreds of contractors on the ground in Syria, helping both the Russian military and pro-regime forces.

Though the question of motive lingers, CNN learned it might have been related to taking control of a vacant oil field:

Valery Shebayev, who visited some of the injured in a Moscow hospital, told CNN the group had been ordered to take what was described as a vacant oil field. But they had no air support. Shebayev, who belongs to a Cossack group from which Wagner draws some of its recruits, described what followed as "a massacre."

So far the Kremlin has been either vague or tight lipped about the incident:

Ruslan Leviev, an activist with the Conflict Intelligence Team in Moscow, a group that monitors Russian involvement in Syria, told CNN that while estimates of the number of varies, "we lean towards 20 to 30 of dead Russian citizens."

The Russian government has declined to confirm the reports. On Thursday, Foreign Ministry spokeswoman Maria Zakharova would only concede that five Russian citizens may have been killed.

But friends and family members of those who died have become impatient and angry with the Kremlin's response.

Among the families of the men who died, there is growing anger. Farkhanur Gavrilova, 67, from the central Russian village of Kedrovoye, lost her son, Ruslan. She only learned of his death from an acquaintance, with no official word from Wagner or the Russian authorities.

Gavrilova contrasted the fate of the Wagner men with state media coverage of Roman Filipov, the pilot shot down by rebels in Syria last month. "Are they not people too? They obviously went to fight, to help, even if it's for the money it's because of poverty, because there are no jobs," Gavrilova told an online private network, Current Time.